Chester S. Knurek

The SCALPEL proof of concept system

Abstract We have designed and constructed a projection electron beam lithography system based on the SCALPEL (SCattering with Angular Limitation in Projection Electron beam Lithography) principle. The experimental tool was built to analyze the efficacy of this approach as an alternative to photolithography for future integrated circuit manufacturing. In this paper we will describe the design of the system and show preliminary results of test pattern exposures. We will show printed features down to 0.08 μm as well as lithographic properties, such as depth of focus, which has been measured at 75 μm for 0.25 μm lines and spaces.

SCALPEL proof-of-concept system: preliminary lithography results

We have designed, constructed, and are now performing experiments with a proof-of-concept projection electron-beam lithography system based upon the SCALPELR (scattering with angular limitation projection electron-beam lithography) principle. This initial design has enabled us to demonstrate the feasibility of not only the electron optics, but also the scattering mask and resist platform. In this paper we report on some preliminary results which indicate the lithographic potential and benefits of this technology for the production of sub-0.18 micrometer features.

200 mm SCALPEL mask development

SCALPEL is a tue 4X reduction technology that capabilities on high resolution capabilities from electron beam exposure and high throughput capabilities from projection printing. Current mask blank fabrication for SCALPEL technology use widely available 100 mm, crystalline silicon wafers. The use of 100 mm crystalline wafers and a wet, through wafer etch process causes the patterned strut width to increase as the wafer is etched and must be accounted for in the mask blank fabrication process. In the wet etch process, a 100 micrometers wide strut grows to 800 micrometers at the strut-membrane interface. As a consequence, the maximum printable die size due to the wafer size and the decreased amount of open area between each strut is 8 X 8 mm. Additionally, crystal defects in the silicon wafer affect the wet etch process and contribute to mask blank failures. A partial solution for an increased die size is to increase the wafer size used to make the SCALPEL mask blank. A 200 mm wafer is capable of producing large die sizes. This can be further improved by dry etching of the grill structure to form struts with vertical sidewalls. As a result, due sizes of 25 X 25 mm or 16 X 32.5 mm can be produced depending on the grill pattern used. However, use of large wafers and dry etching for mask blank formation has significant issues that must be addressed. Among the issues to be addressed are etch chemistries, etch mask materials, and wafer handling.

Space-charge results from the SCALPEL proof-of-concept system

Charged particle lithography systems face a unique challenge because throughput and resolution are linked through the dependence of beam blur on beam current. Understanding the function from of this dependence is vital, both for understanding the throughput limits of such systems, and also for the purposes of optimizing system design. We have developed a simple model describing the effects of image blur on printed resist feature size. The uncertainty in resist feature measurement enables us to determine the overall image blur to an accuracy of approximately 5 nm. We have also begun to develop an aerial image monitoring scheme that can, in principle, measure the image blur to an accuracy of<EQ 1 nm. While the resist based measurement scheme is useful for determining large space-charge blurs, and for optimizing the resist itself, the aerial image monitoring approach has sufficient accuracy to allow us to determine the functional dependence of beam blur on current.

Space-charge limitations to throughput in projection electron-beam lithography (SCALPEL)

Abstract Coulomb interactions in high-throughput charged particle lithography systems lead to an uncorrectable image blur, and can also results in image placement errors. Throughput is ultimately limited by the increasing loss of process latitude with increasing beam current, or by the loss of critical dimension or overlay control due to placement errors. We previously developed an analytical model of the stochastic space-charge induced beam blur for our SCALPEL (SCattering with Angular Limitation Projection Electron Lithography) system, and used it to optimize the design of our experimental tool; principally by constraining the column length and optimizing the numerical aperture to achieve the best balance between electron-optical aberrations and the stochastic blur. We have attempted to validate this model with experimental measurements. We have also begun to quantify pattern distortions due to the global space-charge effects. Preliminary measurements show dose latitudes of ∼ 15%, consistent with a total blur of ∼ 150 nm, and a space-charge component no larger than 30 nm. No evidence for current dependent intrafield distortions has been observed.

SCALPEL mask blank fabrication

The SCALPEL lithography system combines the advantages of high resolution and wide process latitude of electron beam lithography with the throughput of a projection system. The SCALPEL approach has the potential to meet the minimum feature size requirements of future IC generations down to 50 nm.

Production of SCALPEL masks in a commercial mask facility

The purpose of this paper is to investigate the viability of fabricating SCALPEL masks at a DuPont Photomasks, Inc. commercial mask shop. The MEBES 4500 electron beam exposure system and standard inspection tools were used in SCALPEL manufacture to study the key issues to be overcome and the key components needed to succeed in large-scale manufacture. SCALPEL is a next generation lithography technology being researched and developed at Lucent Technologies as the semiconductor industry moves beyond optical lithography. The SCALPEL tool uses a membrane-type mask for high-resolution patterning on Si wafers. SCALPEL mask manufacturing present new and challenging operations in a commercial mask production facility. The production sequence of SCALPEL masks is not uncommon to the current Cr/Qz environment, but introduces the commercial facility to issues at a different level. SCALPEL mask exposure has been accomplished using MEBES III and an advanced MEBES 4500 e-beam lithography system. Pattern imaging, CD metrology, defect inspection, registration metrology, mask handling, and cleaning operations have been attempted with various levels of success. Data and further development of the processes in the commercial facility, along with the challenges and results of these experiences, are detailed in this presentation.

Techniques to inspect SCALPEL masks

As semiconductor lithography nodes become increasingly difficult to achieve with traditional optical lithography, several new technologies have emerged. SCALPEL (SCattering with Angular Limitation Electron beam Lithography) is at the forefront of the NGL technologies. SCALPEL technology uses an electron beam rather than laser light to produce images on the wafer. The SCALPEL mask is non-traditional in the sense that it is silicon-based instead of glass-based and the patterns are written on a membrane. SCALPEL provides unique challenges for the mask maker as well as the semiconductor manufacturer. In this study, we have demonstrated that the KLA-Tencor 3XX platform is capable of inspecting prototype SCALPEL reticles for pattern defects. The inspections were performed with two light wavelengths: 488 nm and 365 nm. Included are the difficulties faced and a projected roadmap for the inspection tool when SCALPEL enters at the 100 nm technology node.

Overview of SCALPEL mask technology

Scattering with angular limitation projection electron beam lithography is a true 4X reduction technology that capitalizes on high resolution capabilities from electron beam exposure and high throughput capability from projection.